However, we find clear evidence that the model breaks down as the
source flux declines. The optical spectra require increasingly
unlikely levels of reprocessing to explain the observed emission. A
change in origin of the optical emission is confirmed by the dramatic
change in optical/X-ray cross-correlation signal [1]}.
While the cross-correlation can be explained in models where the
optical is produced by the jet and the X-rays in a corona [2]},
this does not explain the excellent match between the
optical and X-ray spectra. Instead it seems more likely that there is
a single component connecting the optical and X-ray spectra. The
complex cross-correlation then remains an issue especially for a
single synchrotron component from the jet, but it may potentially be
explained by thermal Comptonisation of IR cyclo-synchrotron emission in
an inhomogeneous hot flow.
